This invention relates to ion implantation.
As is well known, ion implantation is a process of generating an ion beam, focusing that beam, and directing it toward a wafer to implant ions into the wafer.
As fast moving particles in an ion implanter collide with the residual gas and the walls of the implanter, they generate low energy ions and free electrons. A positively charged ion beam traps these electrons and simultaneously rejects the positive ions. The positive ion beam has an inherent potential that is typically distributed nonuniformly across the beam cross-section. (The inherent potential of the beam is also known as the space charge of the beam.)
When the ion beam strikes the wafer surface, low energy electrons are emitted and the wafer tends to become positively charged. Generally, the net amount of positive charge delivered to the wafer is directly proportional to the beam current. In the case of a high current beam, the positive charge on the wafer tends to become quite high - in tens of volts.
When the wafer surface is well grounded to the vacuum enclosure and free of dielectric layers, the charge mainly flows to ground. However, ions are typically implanted after one or more dielectric layers have already been formed on the surface. These layers act as isolated islands on which the ion beam creates electrostatic charge.
The charge build up creates problems. The electrostatic charge interacts with the beam and causes it to lose density, which is a disadvantage because variations in ion beam density results in a nonuniform implantation process. Also, electrostatic charge may discharge and destroy the already formed dielectric layers. With smaller size integrated circuits, the susceptibility of dielectric layers to destruction by such discharge increases. Hence, there is low tolerance for surface charge buildup during ion implantation process.
A solution to these problems is to introduce a neutralizing charge, e.g electrons, to the surface of the wafer and/or to the beam before it contacts the wafer. One implementation of this method is to use a so-called electron shower to supply the neutralizing charge. Another is to use plasma-generating sources to supply low energy electrons and positive ions. Both of these methods typically apply the neutralizing charge near where the beam contacts the wafer.
Electron showers, however, typically supply a large number of high energy electrons which themselves contribute to charging of the wafer surface.
Plasma sources, which typically supply a higher proportion of low energy electrons and ions than do electron showers, not only better neutralize the beam and the surface charge but also contribute less to negative charge buildup on the wafer. When using a plasma source, however, a large plasma density is required to neutralize the beam. The required density can increase the pressure in the vacuum enclosure and degrade the efficiency of the implantation process. Moreover, a uniform and dense plasma is necessary. This is a particularly stringent requirement in respect of scanned beams, because of the wide scanning path. In the case of high current, magnetically scanned beams, the scanning area can be quite large, e.g. approximately 400 mm .times.100 mm.